1. Field of the Invention
The present invention relates to a technology for printing a plurality of images on a sheet by using a print head.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2008-126530 discusses a printing apparatus that performs, by using a long continuous sheet wound in a roll shape, duplex printing on a front and a back (first surface and second surface) of the sheet by an ink jet method. The printing apparatus forms a mark on the first surface and, in the case of printing on the second surface, reads the mark to relatively align images formed on the front and the back.
Though not discussed in Japanese Patent Application Laid-Open No. 2008-126530, there is known a method for carrying out maintenance by preliminarily discharging ink to the sheet from the print head in a periodic or nonperiodic manner to reduce defective discharge of ink caused by nozzle clogging of the print head.
When maintenance of a printing unit is carried out in duplex printing as discussed in Japanese Patent Application Laid-Open No. 2008-126530, the following issues occur.
When defective discharge of ink occurs in the print head when printing of a plurality of images, maintenance must be carried out. An example is a case where an abnormality occurs in the printing unit when printing on the first surface in duplex printing, thus disabling normal printing of some images. In this case, when an area following a defective image of the first surface is used for maintenance, in subsequent printing on the second surface, no image can be printed on areas corresponding to the defective image and the area used for the maintenance. As a result, the areas corresponding to the defective image and the area used for the maintenance must be discarded, which increases wasteful consumption of sheets. The increase in wasteful sheet feeding causes a decrease of total printing throughput.